Electrode wear compensating apparatus for an electrical discharge machine

ABSTRACT

Apparatus for compensating the electrode wear in an electric discharge machine tool wherein an end of electrode travel signal is generated by actuation of a limit switch. The limit switch is actuated by a lead screw which is physically connected to the electrode holding means. The lead screw is moved a predetermined distance away from the limit switch by a motor means on each operation of the electrical discharge machine (EDM).

Unite States Patent Stirner et al.

[ 1 3,688,074 Aug. 29, 1972 [54] ELECTRODE WEAR COMPENSATING APPARATUSFOR AN ELECTRICAL DISCHARGE MACHINE [72] Inventors: Kurt Stirner,Churchville; Allen Berger, Wayne, both of Pa.

[73] Assignee: Jade Corporatlon, Huntingdon Valley, Pa.

22 Filed: July12,1971 21 Appl.No.: 161,812

52 U.S.Cl...; ..2l9/69G,- 219/69V 51 Int. Cl. ..B23p 1/08 58 Field ofSearch ..2l9/69 G, 69 v [56] References Cited UNITED STATES PATENTS3,462,576 8/1969 Ellis ..2l9/69 V Primary Examiner-R. F. StaublyAttorney-Arthur l-l. Seidel et al.

[57] ABSTRACT Apparatus for compensating the electrode wear in anelectric discharge machine tool wherein an end of electrode travelsignal is'generated by actuation of a limit switch. The limit switch isactuated by a lead screw which is physically connected to the electrodeholding means. The lead screw is moved a predetermined distance awayfrom the limit switch by a motor means on each operation of theelectrical discharge machine (EDM).

'llClaims,4DrawingFigures PATENTEfiAue lmz f 3.688.074

/06 /00 9? /02\ 01.; TABLE (94 DEV/SE SQUARE WAVE FREQ. MOTOR on/vsOSCILLATOR DIV/DER T c/Rcu/m 44 COUNTER I Q/ A //2 v /22 0MPARA I20INVENTORS KURT STIRNER ALLEN BERGER DIGITAL SWITCH 7 2 ATTORNEYS machinethereby compensating for the wear or shortening of the electrodes.

Electrical discharge machining, or EDM, is amethod of machiningelectrical current conductingv materials, particularly metals, byutilizing the energy from an electrical discharge from an electrode toerode away the work piece being machined. During the EDM process, theelectrode is gradually worn or consumed. The amount of electrode weardepends upon the type of materials being machined. The amount ofelectrode wear per work piece may be relatively constant in a productionmachining operation. That is, where successive work pieces of the sametype of material are eroded or machined to the same configuration, theamount of electrode wear per work piece will be relatively constant.

Although the electrode wear per work piece in a production machiningoperation may be relatively constant, machine operators in the past wererequired to make a manual adjustment for each new work piece. The totaltime in a day for an operator to make these adjustments for each workpiece was substantial. During the time that an operator is adjusting themachine, the machine is not producing finished work pieces. In summary,the manual adjustment of the machine in the past was tedious,inefficient and costly in terms of lost production. The tediousmeasurement for each work piece also resulted in a significant number oferrors.

The present invention overcomes these problems. By using the presentinvention, the operator is required to make only an initial reading ofthe electrode wear on the first work piece. The operator may then sethis reading of electrode wear into a digital switch. Thepresent'invention then automatically compensates for electrode wear orconsumption for all successive work pieces by enabling the electrodeholding means to advance by the distance set in by the operator on eachsuccessive operation.

In EDM apparatus of the type to which the invention is applicable, theelectrode holding means and the electrodes held therein are advancedtoward the work piece by a servo motor means. An end of travel signal tostop the servo motor means driving the electrode holding means isgenerated by a trip member contacting a limit switch. The trip memberphysically moves with the electrode holding means. Briefly stated, theessence of this invention is to have a lead screw, which is retracted bya predetermined amount on each operation of the machine, operate thelimit switch. The lead screw is mounted through a mounting means whichis mounted to move with the electrode holding means. The lead screw maybe retracted by a suitable motor means such as a stepper motor. Themotor means to retract the lead screw by a predetermined amount may beenergized by an oscillator through motor drive circuitry. The oscillatormay be energized each time the limit switch is contacted by a leadscrew. The oscillator remains energized until a predetermined number ofoscillations or pulses are counted in a counter means. A comparatormeans which receives an input from a digital switch may be used todetect a predetermined count in the counter means and de-ene'rgize theoscillator. Additional members and limit switches are provides tolimitthe compensatingadjustment for electrode wear to predeterminedamounts.

For the purpose of illustrating the invention, there are shown in thedrawings forms which are presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is an elevation view, partly in cross section, of a' portion ofan electrical discharge machine incorporating an embodiment of thepresent invention.

FIG. 2 is a view, partly in cross section, taken along the line 2-2 ofFIG. 1.

FIG. 3 is an elevation view of a control panel of the present invention.

FIG. 4 is a schematic diagram in block diagram form of the presentinvention.

Referring now to the drawings in detail, wherein like numerals indicatelike elements, there is shown in FIG. 1 portions of an electricdischarge machine having a base member 10. A frame 12 is mounted on basemember 10. The frame 12 may contain servo motor means (not shown) whichare well known in the prior art for driving ram 14. Electrode holdingmember 16 is physically connected to ram 14 through connecting member 18by means of bolts 20 and 22, or any other suitable fastening means wellknown in the-art.

Guide members 24 and 26 are slidably mounted through base 10 andphysically connected to electrode holding member 16 by any suitablemeans well known in the art. Electrode holding member 16 may be providedwith electrodes (not shown) extending vertically downward from electrodeholding member 16. The electrode holding member 16 carrying theelectrodes (not shown) moves vertically in the directions shown by arrow28 in order for the electrodes to engage a work piece (not shown)located in a dielectric liquid medium contained in a tank (not shown) asis well known in the art.

Mounting member 30 is mounted on the upper end of guide member 26.Mounting member 30 is more clearly shownin FIG. 2. As may be moreclearly understood by taking FIG. 2 in connection with FIG. 1, mountingmember 30 may be physically mounted on the upper end of guide member 26by means of a bolt 32 although any other suitable fastening means suchas welding may be employed. Lead screw 34 is threadedly mounted throughmounting member 30. L-shaped member 36 has its longer leg 38 slidablymounted through mounting member 30. The shorter leg 40 of L- shapedmember 36 has the upper end of lead screw 34 rotatably mounted throughit by means of ball bearings 42 as may be seen in the broken-out sectionof the shorter leg 40 of L-shaped member 36. A motor 44 which maypreferably be a stepper motor is securely mounted to the shorter leg 40of L-shaped member 36 by any suitable means such as bolts (not shown).Although L-shaped member 36 is shown as being formed as a unitaryelement, it is understood that the bolting together of two elements suchas the shorter leg 40 and the longer leg 38 would be an equivalent.

A limit switch 46 is mounted on base member 10. Limit switch 46 iscooperatively engaged'by the lower end 48 of lead screw 34 during thedownward travel of electrode holding member 16 and guide member 26.Limit switch 46 creates an end of travel signal which stops the motor(not shown) driving ram 14. As may be seen from FIG. 1, rotation of leadscrew 34 in a counterclockwise direction by motor 44 causes lead screw34 to move in an upward direction with respect to mounting member 30 orin other words to be retracted. This retraction or upward movement oflead screw 34 with respect to mounting member 30 allows guide member 26and electrode holding member 16 to travel farther in a downwarddirection before limit switch 46 is contacted. Therefore, apredetermined amount of rotation in a counterclockwise direction bymotor 44 causes lead screw 34 to be retracted from mounting member 30andprovides a predetermined increment of farther downward travel oradvance.

Members 50 and 52 are mounted on longer leg 38 of L-shaped member 36 bythumbscrews 54 and 56, respectively. Limit switches 58 and 60 aremounted on the upper and lower sides of mounting member 30,respectively. Member 50 contacts limit switch 58 and prevents furtherenergization of motor 44 when lead screw 34 has been sufficientlyrotated in a clockwise direction. That is, limit switch 58 is engaged bymember 50 so when lead screw 34 has been screwed down through mountingmember 30. Limit switch 60 will be engaged by member 52 when lead screw34 has been retracted or screwed up through mounting member 30 after anumber of operations of the electrical discharge machine. In normaloperation, member 52 will be adjusted by thumbscrew 56 on the longer leg38 of L-shaped member 36 so as to prevent further downward adjustment ofelectrode holding member 16 when the electrodes have been worn down totheir minimum usable length.

It is to be understood that the mounting of the various limit switchesmay be varied within the scope of the invention and perform theirfunction equally well. For example, the limit switch 46 may be mountedon end 48 of lead screw 34. In such a case limit switch '46 would beengaged by contacting base 10. The limit switch 58 may be mountedequally well on member 50. The limit switch 58 in such a case would beengaged by contacting mounting member 30. Similarly, limit switch 60could be mounted on member 52.

Referring now to FIG. 3, there is shown a control panel 70 used insetting up the compensating apparatus of the present invention. A digitswitch 72 is provided which may be of the thumb switch type. Thumbswitch 72 has a unit section 74, a tenths section 76 and a hundrethssection 77. In a preferred embodiment of this invention, any numberbetween 000 and 999 may be set into digital switch 72 corresponding toan advance per operation of electrode holding member 16 of zerothousandths of an inch to 999 thousandths of an inch. The number dialedinto digit switch 72 by the operator is determined from a measurement ofthe number of thousandths of an inch of electrode wear on the first workpiece.

A push button 80 is provided on control panel 70 for manual jog upoperation of lead screw 34 which may be necessary in initial set upoperation. Push button 82 is provided for jog down operation of leadscrew 34 during a set up operation. That is, push buttons and 82 areused to manually energize motor 44, overriding the setting on digitalswitch 72, to cause lead screw 34 to rotate and be driven'either up ordown with respect to mounting member 30. A switch 84 is provided toprevent motor 44 from operating when switch 84 is in the hold position.The run position of switch 84 is the normal operating position. During aset up condition,

operation of motor 44 may be prevented by placing switch 84 in the holdcondition even though end 48 of lead screw 34 is in contact with limitswitch 46.

Referring now to FIG. 4, a schematic diagram partially in block diagramform, of the control circuitry of the present invention is shown. Abi-stable device which may be a flip-flop is provided. Bi-stable device90 has a set input 92 and a reset input 94. Output 96 of bi-stabledevice 90enables energization of square wave oscillator 98 whenbi-stable device 90 is in a set condition. Input 92 of bi-stable device90 is provided with a set signal when any one of the inputs 100, 102 or104 of OR gate 106 is provided with a signal. Input terminal 100 of ORgate 106 is provided with a signal each time end 48 of lead screw 34engages limit switch 46. Input leads 102 and 104 of OR gate 106 receivean input signal when push buttons 80 and 82, respectively, aredepressed.

The output of square wave oscillator 98 is fed to frequency divider 108.Frequency divider 108 is used to coordinate the pitch of the threads oflead screw 34 with the steps per revolution of stepper motor 44 so thatthe setting on digit switch 72 is read directly in thousandths of aninch. The output of frequency divider 108 is fed to switching circuit110 and counter l 12. Switching circuit 1 10 provides output pulses byan output line 114 whenever a signal is present on input leads 100 or102 of OR gate 106. Pulses on line 114 cause motor drive circuitry 116to drive stepper motor 44 in a counterclockwise direction causing leadscrew 34 to be retracted or screwed up through mounting member 30.Pulses are provided on line 118 when a signal is present on input 104 ofOR gate 106. When pulses are present on line 118, motor drive circuitry11.6 causes stepper motor 44 to rotate in a clockwise direction therebycausing lead screw 34 to be screwed down through mounting member 30. Thecount of counter 12 is compared in comparator 120 with the setting ondigital switch 72. When coincidence is detected by comparator 120, anoutput signal is generated on line 122 causing bi-stable device 90 to bereset through OR gate 124. The resetting of bi-stable device 90 causessquare wave oscillator 98 to cease oscillation. Therefore, square waveoscillator 98 will oscillate only until stepper motor 44 has caused leadscrew 34 to retract by a predetermined distance determined by thesetting on digital switch 72. Inputs 126, 128 and 130 are fedrespectively with signals from upper limit switch 58, lower limit switch60 and switch 84 in the hold position in order to prevent oscillation ofsquare wave oscillator 98 during the various conditions.

In operation, members 50 and 52 would be adjusted a predetermineddistance apart on longer leg 38 of L- shaped member 36 depending uponthe usable portion of electrodes being employed. After operation on thefirst work piece, a measurement of electrode wear would be made. Thiselectrode wear measurement would be set into digital switch 72 expressedin thousandths of an inch. Jog down button 82 would be depressed untillead screw 34 was screwed down through mounting member 30 until member50 contacted limit switch 58. The machine would then be set up foroperation. Each time end 48 of lead screw 34 contacted limit switch 46,a signal would be felt on input terminal 100 of OR gate 106 therebyenergizing square wave oscillator 98. Assuming as an example that theelectrode wear measurement on the first piece was 50 thousandths of aninch and that 050 was set in to digital switch 72, square waveoscillator 98 would oscillate until comparator 120 detected a count offifty in counter 1 12. Comparator 1-20 via line 122 and OR gate 124would reset bi-stable device 90 thereby turning off square waveoscillator 98. This predetermined period of oscillation of square waveoscillator 98 would have caused lead screw 34 to have been retracted byfifty thousandths of an inch and would therefore allow electrode holdingmember 16 to advance fifty, thousandths of an inch farther on the nextoperation. Thus, electrode holding member 16 would be advanced fiftythousandths of an inch farther on each successive operation of theelectric discharge machine until member 52 contacted limit switch 60. Atthis time, new electrodes would be required.

It will be apparent to those skilled in the art that the wearcompensating apparatus described herein may be used for compensating forwear in various types of cutting and erosion machinery. It is alsoapparent that various changes and modifications may be made in thecircuitry, or different devices used to produce the same. function inthe circuit. For example, the limit switches may be mounted in otherpositions as discussed above.

A rack and pinion may be provided in place of the axially driven leadscrew 34. Mounting means 30 and hold compensating apparatus need not bemounted on top of guide member 26, but may be mounted in any suitablemanner so .long as its physical movement is tied to the physicalmovement of electrode holding member 16. With respect to the circuitry,frequency divider 108 could be eliminated by providing an adjustablesquare wave oscillator. Further, it is not necessary that a square waveoscillator be used, any suitable oscillator may be provided, such as asine wave oscillator.

In view of the above, the present invention may be embodied in otherspecific forms without departing from the spirit or essential attributesthereof and, accordingly, reference should be made to the appendedclaims, rather than to the foregoing specification as indicating thescope of the invention.

We claim:

1. Apparatus for compensating for a decrease in length of an electrodein an electrical discharge machine wherein said electrode carried by anelectrode holding means is reciprocably mounted to advance into andretract out of engagement with a work piece, comprising:

a limit switch;

a lead screw mounted to engage said limit switch when said electrodeholding means advances into engagement with the work piece;

motor means for moving said lead screw in a direction away from saidlimit switch; and

control means for energizing said motor means to move said lead' screwin a direction away from said limit switch by a predetermined distanceeach time said lead screw engages said limit switch.

2. Apparatus for compensating for a decrease in length of an electrodein an electrical discharge machine wherein said electrode carried by anelectrode holding means is reciprocably mounted to advance into andretract out of engagement with a work piece, comprising:

a lead screw threadably mounted through said mounting member;

motor means for rotating said lead screw;

a limit switch stationarily mounted to be engaged by one end of saidlead screw when said electrode holding means approaches its mostadvanced position; and

control means for energizing said motor means to rotate said lead screwby a predetermined amount each time said end ofsaid lead screw contactssaid limit switch thereby allowing said electrode holding means toadvance a predetermined distance on the next operation.

3. Apparatus for compensating for a decrease in length of an electrodeas recited in claim 2 wherein said motor means is a stepper motor.

4. Apparatus for compensating for a decrease in length of an electrodeas recited in claim 3 wherein said control means comprises:

oscillator means for generating pulses;

means for energizing said oscillator means when said end of said leadscrew engages said limit switch;

motor drive means for receiving the pulses generated by said oscillatormeans and driving said stepper motor in response to said pulses; and

means for counting the pulses generated by said oscillator means and forde-energizing said oscillator means when a predetermined number ofpulses is counted.

5. Apparatus for compensating for a decrease in length of a consumableelectrode in an electrical discharge machine having a base and anelectrode carried by a holding means reciprocably mounted to advanceinto and retract out of engagement with a work piece, a guide memberhaving a first and a second end, said guide member mounted at said firstend to said holding means and slidably mounted through said base,comprising:

a mounting member mounted on said second end of said guide member;

a lead screw threadably mounted through said mounting member;

an L-shaped member, the longer leg of said L-shaped member beingslidably mounted through said mounting member and substantially parallelto said lead screw, said lead screw being rotatably mounted to theshorter leg of said L-shaped member with said l e-shaped memberconstrained to follow axial movement of said lead screw, said L-shapedmember slidably moving through said mounting member in response tomovement of said lead'screw;

motor means for rotating said lead screw;

a first limit switch mounted on said base, one end of said lead screwengaging said first limit switch when said holding means approaches itsmost advanced position; and

control means for energizing said motor means to rotate said lead screwby a predetermined amount each time said end of said lead screw engagessaid first limit switch thereby allowing said holding means to beadvanced a predetermined distance on the next operation.

6. Apparatus for compensating for a decrease in length of an electrodeas recited in claim including:

a first and a second member mounted a predetermined distance apart onsaid longer leg of said L- shaped member; and

a second and-a third limit switch mounted along the path of said longerleg of said L-shaped member, said second and said third limit switchesbeing engaged by said first and said second members,

length of an electrode as recited in claim 5 wherein said motor means isa stepper motor.

9. Apparatus for compensating for a decrease in length of an electrodeas recited in claim 8 wherein said control means comprises:

oscillator means for generating pulses;

means for'energizing said oscillator means when said end of said leadscrew engages said first limit switch;

motor drive means for receiving the pulses generated by said oscillatormeans and driving said stepper motor in response to said pulses; and

means for counting the pulses generated by said oscillator means and forde-energizing said oscillator means when a predetermined number ofpulses is counted.

10. Apparatus for compensating for a decrease in length of an electrodeas recited in claim 9 wherein said counting'means includes a digitalswitch to vary the predetermined number to which said counting meanswill count before de-energizing said oscillator means.

11. Apparatus for compensating for a decrease in length of an electrodeas recited in claim 10 wherein said oscillator means includes afrequency divider.

1. Apparatus for compensating for a decrease in length of an electrodein an electrical discharge machine wherein said electrode carried by anelectrode holding means is reciprocably mounted to advance into andretract out of engagement with a work piece, comprising: a limit switch;a lead screw mounted to engage said limit switch when said electrodeholding means advances into engagement with the work piece; motor meansfor moving said lead screw in a direction away from said limit switch;and control means for energizing said motor means to move said leadscrew in a direction away from said limit switch by a predetermineddistance each time said lead screw engages said limit switch. 2.Apparatus for compensating for a decrease in length of an electrode inan electrical discharge machine wherein said electrode carried by anelectrode holding means is reciprocably mounted to advance into andretract out of engagement with a work piece, comprising: a lead screwthreadably mounted through said mounting member; motor means forrotating said lead screw; a limit switch stationarily mounted to beengaged by one end of said lead screw when said electrode holding meansapproaches its most advanced position; and control means for energizingsaid motor means to rotate said lead screw by a predetermined amounteach time said end of said lead screw contacts said limit switch therebyallowing said electrode holding means to advance a predetermineddistance on the next operation.
 3. Apparatus for compensating for adecrease in length of an electrode as recited in claim 2 wherein saidmotor means is a stepper motor.
 4. Apparatus for compensating for adecrease in length of an electrode as recited in claim 3 wherein saidcontrol means comprises: oscillator means for generating pulses; meansfor energizing said oscillator means when said end of said lead screwengages said limit switch; motor drive means for receiving the pulsesgenerated by said oscillator means and driving said stepper motor inresponse to said pulses; and means for counting the pulses generated bysaid oscillator means and for de-energizing said oscillator means when apredetermined number of pulses is counted.
 5. Apparatus for compensatingfor a decrease in length of a consumable electrode in an electricaldischarge machine having a base and an electrode carried by a holdingmeans reciprocably mounted to advance into and retract out of engagementwith a work piece, a guide member having a first and a second end, saidguide member mounted at said first end to said holding means andslidably mounted through said base, comprising: a mounting membermounted on said second end of said guide member; a lead screw threadablymounted through said mounting member; an L-shaped member, the longer legof said L-shaped member being slidably mounted through said mountingmember and substantially parallel to said lead screw, said lead screwbeing rotatably mounted to the shorter leg of said L-shaped member withsaid L-shaped member constrained to follow axial movement of said leadscrew, said L-shaped member slidably moving through said mounting memberin response to movement of said lead screw; motor means for rotatingsaid lead screw; a first limit switch mounted on said base, one end ofsaid lead screw engaging said first limit switch when said holding meansapproaches its most advanced position; and control means for energizingsaid motor means to rotate said lead screw by a predetermined amounteach time said end of said lead screw engages said first limit switchthereby allowing said holding means to be advanced a predetermineddistance on the next operation.
 6. Apparatus for compensating for adecrease in length of an electrode as recited in claim 5 including: afirst and a second member mounted a predetermined distance apart on saidlonger leg of said L-shaped member; and a second and a third limitswitch mounted along the path of said longer leg of said L-shapedmember, said second and said third limit switches being engaged by saidfirst and said second members, respectively, to prevent advance andretraction of said holding means beyond predetermined limits. 7.Apparatus for compensating for a decrease in length of an electrode asrecited in claim 6 wherein the position of at least one of said firstand second members on said longer leg of said L-shaped member may beadjusted.
 8. Apparatus for compensating for a decrease in length of anelectrode as recited in claim 5 wherein said motor means is a steppermotor.
 9. Apparatus for compensating for a decrease in length of anelectrode as recited in claim 8 wherein said control means comprises:oscillator means for generating pulses; means for energizing saidoscillator means when said end of said lead screw engages said firstlimit switch; motor drive means for receiving the pulses generated bysaid oscillator means and driving said stepper motor in response to saidpulses; and means for counting the pulses generated by said oscillatormeans and for de-energizing said oscillator means when a predeterminednumber of pulses is counted.
 10. Apparatus for compensating for adecrease in length of an electrode as recited in claim 9 wherein saidcounting means includes a digital switch to vary the predeterminednumber to which said counting means will count before de-energizing saidoscillator means.
 11. Apparatus for compensating for a decrease inlength of an electrode as recited in claim 10 wherein said oscillatormeans includes a frequency divider.